The present application is based on Japanese Patent Application No. 11-107866 filed Apr. 15, 1999, the contents of which are incorporated hereinto by reference.
1. Field of the Invention
The present invention relates to an improvement of semiconductor surface-emitting elements such as a light-emitting diode (LED) and a vertical cavity surface-emitting laser (VCSE).
2. Discussion of the Related Art
There has been known a semiconductor surface-emitting element including a plurality of semiconductor layers which are laminated on a semiconductor substrate, wherein the plurality of semiconductor layers include at least one light-generating layer of a quantum well structure which generates a light and has a thickness value smaller than the wavelength (100 xc3x85, i.e., 10 nm) of the electron wave, and a light resonator consisting of a pair of reflecting layers which are located on the opposite sides of the at least one light-generating layer, for reflecting the light generated by the at least one light-generating layer. By applying an electric current between two electrodes respectively formed on opposite two major surfaces of a laminar structure consisting of the plurality of semiconductor layers, the light generated by the at least one light-generating layer is emitted from one of the two opposite major surfaces of the laminar structure which is remote from the substrate. In the thus constructed semiconductor surface-emitting element called as a resonant-cavity light-emitting diode (RC-LED), for instance, the electron wave in the light-generating layer and the optical wave in the light resonator are coupled together, so that the at least one light-generating layer generates a light only in a resonance mode. This phenomenon is generally referred to as cavity QED effect. Owing to this cavity QED effect, the above-described semiconductor surface-emitting element is capable of emitting a light which has a high degree of directivity and a narrow line width, so that the emitted light does not suffer from total reflection on the crystal face, assuring a high degree of external quantum efficiency. One example of such a semiconductor element is disclosed in JP-A-4-167484.
The above-described semiconductor surface-emitting element having the light resonator in which there is disposed the at least one light-generating layer of a quantum well structure is capable of assuring a high degree of external quantum efficiency. This semiconductor surface-emitting element, however, emits a light in which a wavelength width over which the gain is obtained is relatively small. In other words, the wavelength width over which the gain ranging from 80% to 90% is obtained at a peak wavelength of an emission spectrum of the element is relatively small.
In the light-generating layer of a quantum well structure, the peak wavelength of its emission spectrum tends to vary with a change of the operating temperature of the semiconductor surface-emitting element. In this case, if the amount of change of the operating temperature of the surface-emitting element is relatively large, the resonance wavelength of the light resonator undesirably deviates from the wavelength range of the emission spectrum of the light-generating layer in which the gain is obtained. Accordingly, the operating temperature range wherein the semiconductor light-emitting element is operated is inevitably limited to a specific range in which the element emits a light having a desired emission wavelength. In view of this, there has been proposed another type of semiconductor surface-emitting element wherein a plurality of light-generating layers having different bandgap sizes, namely, different emission wavelengths, are provided within the light resonator. One example of such a semiconductor surface-emitting element is disclosed in JP-A-10-27945. The disclosed element generates an optical output with high stability even when the element is operated at a relatively high temperature since the disclosed element is capable of providing a certain level of gain irrespective of a rise of the operating temperature of the element.
However, the above-described publication merely discloses a semiconductor surface-emitting element which includes a plurality of light-generating layers having different emission wavelengths within the light resonator. In the thus constructed semiconductor surface-emitting element, the optical output of the element may fluctuate in the operating temperature range of the element, e.g., in the range of xe2x88x9240xc2x0 C.xcx9c+105xc2x0 C. Accordingly, the disclosed element does not always generate a stable optical output at any temperature value within the operating temperature range of the element.
The present invention was developed in the light of the above-described situation. It is therefore an object of the present invention to provide a semiconductor surface-emitting element having a light reflector in which there is disposed a plurality of light-generating layers of a quantum well structure, which semiconductor surface-emitting element is capable of generating an optical output with high stability within an operating temperature range of the element.
The above-described object of the present invention may be attained according to a principle of the invention, which provides a semiconductor surface-emitting element having a light-emitting portion and formed of a plurality of semiconductor layers including a plurality of light-generating layers of a quantum well structure for generating lights having respective different emission spectra whose peak wavelengths are different from one another, and a pair of reflecting layers between which the plurality of light-generating layers are interposed for reflecting the lights generated by the plurality of light-generating layers, the pair of reflecting layers functioning as a light resonator, the semiconductor surface-emitting element emitting a light resonated by the light resonator from the light-emitting portion, wherein the improvement comprises: one of the plurality of light-generating layers which has an emission spectrum having a shortest peak wavelength being formed such that the shortest peak wavelength is substantially equal to a light resonance wavelength of the light resonator at a predetermined maximum operating temperature of the semiconductor surface-emitting element, while another of the plurality of light-generating layers which has an emission spectrum having a longest peak wavelength being formed such that the longest peak wavelength is substantially equal to the light resonance wavelength of the light resonator at a predetermined minimum operating temperature of the semiconductor surface-emitting element.
In the semiconductor surface-emitting element constructed according to the present invention wherein the plurality of light-generating layers have respective different emission spectra whose peak wavelength are different from one another, the light-generating layer which has the emission spectrum having the shortest peak wavelength is formed such that the shortest peak wavelength is substantially equal to the light resonance wavelength of the light resonator at the predetermined maximum operating temperature of the semiconductor surface-emitting element, while the light-generating layer which has the emission spectrum having the longest peak wavelength is formed such that the longest peak wavelength is substantially equal to the light resonance wavelength of the light resonator at the predetermined minimum operating temperature of the element. In the thus constructed semiconductor surface-emitting element, the wavelength width of the light emitted from the element over which a gain is obtained is sufficiently large at any temperature value within the operating temperature range of the element, so that the surface-emitting element is capable of generating an optical output with high stability within the operating temperature range.
In one preferred form of the present invention, the plurality of light-generating layers are formed of the same compound semiconductor and have respective different thickness values. According to this arrangement, the plurality of light-generating layers having the respective different emission wavelengths are easily formed by controlling a time duration for which each light-generating layer is grown, such that each light-generating layer has the desired thickness value.
In another preferred form of the present invention, the plurality of light-generating layers have the same thickness value and are formed of respective compound semiconductors all of which consist of a same compound and which have respective different proportions of components of the compound. According to this arrangement, the plurality of light-generating layers having the respective different emission wavelengths can be easily formed by suitably controlling a switch valve device for adjusting the proportions of the components of the compound of the compound semiconductor which gives each light-generating layer during the crystal growing process of each light-generating layer.
In still another preferred form of the present invention, the pair of reflecting layers consists of a first reflecting layer and a second reflecting layer, the first reflecting layer consisting of a multiplicity of unit semiconductors which are formed by crystal growth on a monocrystalline GaAs substrate and which constitute a first distributed-Bragg reflector located on the side of the substrate, the plurality of light-generating layers of a quantum well structure including at least two light-generating layers each of which is interposed between two semiconductor barrier layers which are formed by crystal growth, the second reflecting layer consisting of a multiplicity of unit semiconductors which are formed by crystal growth on an upper one of the two barrier layers between which an uppermost one of the at least two light-generating layers is interposed, the multiplicity of unit semiconductors of the second reflecting layer constituting a second distributed-Bragg reflector located on the side of the light-emitting portion. According to this arrangement, the first reflecting layer, the barrier layers, the light-generating layers, and the second reflecting layer are grown on the substrate within a single processing chamber by suitably controlling a switch valve device.
In yet another preferred form of the present invention, the plurality of light-generating layers are formed in the semiconductor surface-emitting element such that each of the plurality of light-generating layers is aligned with a corresponding one of at least one antinode of a standing wave generated in the light resonator. This arrangement effectively increases the quantum efficiency of the surface-emitting element, resulting in a significantly high degree of light emission output of the element.